The present invention is directed to a stage assembly for moving a device. More specifically, the present invention is directed to a long stroke mover for moving a stage of the stage assembly.
Exposure apparatuses are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that retains a reticle, a lens assembly and a wafer stage assembly that retains a semiconductor wafer. The reticle stage assembly and the wafer stage assembly are supported above a ground with an apparatus frame.
Typically, the wafer stage assembly includes a wafer stage base, a wafer stage that retains the wafer, a wafer vacuum preload type air bearing that supports the wafer stage, and a wafer mover assembly that precisely positions the wafer stage and the wafer. Somewhat similarly, the reticle stage assembly includes a reticle stage base, a reticle stage that retains the reticle, a reticle vacuum preload type air bearing that supports the reticle stage and a reticle mover assembly that precisely positions the reticle stage and the reticle. Typically, the wafer vacuum preload type air bearing is created by releasing air from outlets in a bottom of the wafer stage towards the wafer stage base and pulling a vacuum in inlets in the bottom of the wafer stage. Similarly, the reticle vacuum preload type air bearing is created by releasing air from outlets in a bottom of the reticle stage towards the reticle stage base and pulling a vacuum in inlets in the bottom of the reticle stage.
The size of the images and features within the images transferred onto the wafer from the reticle are extremely small. Accordingly, the precise positioning of the wafer and the reticle relative to the lens assembly is critical to the manufacture of high density, semiconductor wafers.
Depending upon the type of energy beam generated by the illumination source, the type of fluid surrounding the reticle and the wafer can influence the performance of the exposure apparatus. For example, some types of beams, e.g. electron beams and very short wavelengths of ultraviolet light, are absorbed by oxygen and other gases. As is well known, air is a gaseous mixture that is approximately twenty-one percent oxygen. Thus, air surrounding the reticle and wafer can influence the performance of the exposure apparatus and the quality of the integrated circuits formed on the wafer can be compromised. As a result thereof, the performance of the exposure apparatus and the quality of the integrated circuits formed on the wafer can be enhanced by controlling the environment around one or both stages.
One way to control the environment around a stage is to position a chamber around the stage. Subsequently, the desired environment can be created within the chamber around the stage. For example, the chamber may be filled with an inert fluid. Alternately, some processes require that the controlled environment is a vacuum.
Historically, stage assemblies used in a vacuum environment have utilized mechanical type bearings to support the stage. Typical mechanical type bearings include ball bearings, roller bearings or sliding contact. However, limitations on the use of lubricants in a vacuum, rolling or sliding noise or vibration, particle generation, and friction also limit the accuracy and velocity of such stages.
Another solution is to use an air bearing in the vacuum to support the stage. However, air bearings typically require substantial preload forces to have high stiffness, which is desirable for precision stages. Unfortunately, it is not possible to create a vacuum preload type air bearing if the stage is surrounded by a vacuum.
Alternately, a lower air bearings and an opposed upper air bearing can be used to support the stage in the vacuum environment. In this embodiment, the upper air bearing preloads the lower air bearing to create a relatively stiff bearing. However, this design requires an increase in stage mass and/or complexity and an increase in the number of air bearings required by the stage assembly. In addition, the opposed air bearings can deform the stage. Further, the air released to create the air bearing that supports the stage is also released into the chamber. This can compromise the vacuum that is created within the chamber. Thus, the use of air bearings to support the stage can make it difficult to control the environment around the stage.
Additionally, depending upon the type of energy beam generated by the illumination source, the motors used to move the stages can influence the performance of the exposure apparatus. For example, a typical brushless electric motor includes one or more magnets and one or more coils. Unfortunately, the magnetic fields from the motor can influence a number of manufacturing, measurement and/or inspection processes. More specifically, for example, electron beams are influenced by magnetic fields of sufficient magnitude. As a result thereof, the electric motors must be positioned a relatively large distance away from the electron beam. Similar design considerations apply to other charged particle lithography systems, including ion beam systems, as well as charged particle inspection or metrology systems.
In light of the above, there is a need for a bearing assembly and method for supporting a stage that does not compromise the desired environment around the stage. Additionally, there is a need for a relatively stiff bearing assembly for supporting a stage in a vacuum. Moreover, there is a need for a bearing assembly for an exposure apparatus that utilizes an electron beam. Further, there is a need for a mover that can be positioned relatively close to a stage while reducing the influence of the motor on the energy beam. Also, there is a need for an exposure apparatus capable of manufacturing precision objects, such as high density, semiconductor wafers.
The present invention is directed to a stage assembly that moves a device. The stage assembly includes a guide base, a stage that retains the device, and a stage bearing assembly that supports the stage spaced apart from the guide base. Uniquely, the stage bearing assembly generates an electrostatic force that urges the stage towards the guide base. Typically, an opposing force is required to balance the electrostatic force. The opposing force can be provided by a number of alternate ways. Further, the electrostatic force can be modulated based on position feedback to provide stiffness and damping relative to a reference. As a result of this design, the stage bearing assembly can be relatively stiff without gas leakage or mechanical contact. Thus, the stage bearing assembly is particularly useful in manufacturing, measurement and/or inspection processes that are operated in a controlled environment such as a vacuum.
As provided herein, the stage bearing assembly includes a base conductive section and a stage conductive assembly that cooperate to generate the electrostatic force that urges the stage towards the guide base. Further, the stage conductive assembly includes a first stage conductive section, a second conductive section and a third conductive section. Moreover, each of the stage conductive sections is electrically isolated from the other stage conductive sections.
The stage assembly also includes a control system that controls the voltage to each of the conductive sections. As provided herein, the control system controls the voltage to the stage conductive sections to actively adjust the force generated by the stage bearing assembly. Preferably, the control system individually controls the voltage to each of the stage conductive sections to adjust the position of the stage relative to the guide base along a Z axis, about an X axis and about a Y axis.
As an overview, the electrostatic attractive pressure between two conductive surfaces is proportional to the voltage difference squared divided by the gap squared. The electrostatic repulsive forces caused by common voltage are very low and are therefore not used by the control system to control the stage bearing assembly. Because only attractive electrostatic forces are generated, an opposing force is needed to balance the electrostatic forces. Techniques for providing the opposing force will be presented in the embodiments below.
Preferably, the stage assembly includes a gap measuring device for measuring a gap between the base conductive section and each of the stage conductive sections and providing feedback of the position of the stage relative to the guide base. The gap measuring device can be used to calculate the voltage required to yield the desired electrostatic force. More specifically, the non-linear and unstable nature of the electrostatic force can be linearized in the control system using the gap measurement in the following equations:
V=(h gap+h dielectric/k dielectric){square root over ( )}(2F/xcex50A)
where: h gap=mechanical clearance and h dielectric=dielectric thickness, k dielectric=dielectric constant, F=desired force, xcex50=permittivity constant=8.854xc3x9710xe2x88x9212 Farad/m, A=area.
Thus, the voltage required to generate the desired force is calculated using the gap between the base conductive section and each of the stage conductive sections. In one embodiment, the gap measuring device includes a capacitance sensor that measures the capacitance between the base conductive section and each of the stage conductive sections. Using the position feedback, the electrostatic force generated by the stage bearing assembly can be controlled and stiffness of the stage bearing assembly can be created relative to the base.
A couple of embodiments of the stage assembly are provided herein. In a first embodiment, the guide base is positioned above the stage and the electrostatic force generated by the stage bearing assembly urges the stage upward towards the guide base. In the first embodiment, gravity on the stage provides a constant downward force that opposes the attractive electrostatic force. Therefore, the net vertical force at each electrostatic conductive area on the stage can be made positive or negative by controlling the electrostatic attractive force to be greater than or less than the force of gravity.
Preferably, in this embodiment, the stage assembly further includes a safety stop positioned below the stage, so that the stage is prevented from falling a large distance if the voltage is removed from the stage bearing assembly. As provided herein, the safety stop includes a safety plate and a safety mover assembly. The safety plate catches the stage in the event the stage bearing assembly fails. The safety mover assembly moves the safety plate towards the guide base and lifts the stage until the gap is sufficiently small so that the stage bearing assembly can support the stage, at which point the stage plate lowers to prevent contact with the stage.
Alternately, the safety stop can be fixed in height and the guide base is lowered down until the gap is sufficiently small so that the stage bearing assembly can support the stage. Subsequently, the guide base is raised back to its operating position.
Still alternately, in the second embodiment, the guide base is positioned below the stage and the electrostatic force generated by the stage bearing assembly urges the stage downward towards the guide base. Further, in this embodiment, the stage bearing assembly includes a relatively low stiffness fluid bearing that urges the stage upward and away from the guide base in order to balance the electrostatic attractive force. In this design, the overall bearing stiffness is again provided by actively controlling the electrostatic force.
Additionally, in each embodiment, the stage assembly preferably includes a stage mover assembly connected to the stage that moves the stage relative to the guide base. The stage mover assembly can move the stage along an X axis, along a Y axis and about a Z axis relative to the guide base. As provided herein, the stage mover assembly includes a Y mover that moves the stage along the Y axis. The Y mover includes a magnet that is substantially fixed relative to the guide base and a conductor that is secured to the stage. The magnet has a magnet length that extends along the X axis and the conductor has a conductor length along the X axis.
Preferably, the magnet length is greater than the combination of the length of the conductor and the length of the stroke of the stage mover assembly along the X axis. This design of the Y mover eliminates the need to move the magnets of the Y mover as the stage is moved along the X axis. Stated another way, the magnets of the Y mover remain substantially in the same position during movement of the stage along the X axis. Because magnetic fields from the magnets can influence the energy beam from a charge particle exposure apparatus, it is preferable to maintain the magnets in a fixed position so that the influence from the magnets is constant and can be compensated. Thus, the mover provided herein is particularly useful in manufacturing, measurement and/or inspection processes that are sensitive to and/or influenced by stray changing magnetic fields.
The present invention is also directed to a method for making a stage assembly, a method for making an exposure apparatus and a method for making an object utilizing the exposure apparatus.